1. Field of the Invention
The present invention relates to a substrate cleaning device for cleaning a surface of a substrate processed by, for example, an emersion exposure process or a developing process, and a substrate cleaning method to be carried out by the substrate cleaning device.
2. Description of the Related Art
A photoresist pattern forming process, which is one of semiconductor device fabricating processes, coats a surface of a semiconductor wafer (hereinafter, referred to simply as “wafer”) with a resist film, exposes the resist film in a predetermined pattern, and develops the exposed resist film to form a resist pattern.
Device patterns have been progressively miniaturized and the thickness of films shows a trend to decrease in recent years. Consequently, there has been a growing request for higher exposure resolution. To improve exposure resolution, efforts have been made to develop exposure techniques, such as an exposure method using extreme ultraviolet rays (EUVL), an exposure method using electron beam projection (EPL) and an exposure method using a fluorine dimer (F2). On the other hand, an immersion exposure method has been studied to achieve higher resolution through the further improvement of exposure techniques using a conventional light source, such as a light source of argon fluoride (ArF) or krypton fluoride (KrF). The immersion exposure method passes light through, for example, pure water to use the effect of water to shorten the wavelength of radiation emitted by an ArF light source from 193 nm to 134 nm. The immersion exposure method can achieve high resolution by using an existing light source.
The immersion exposure method will be briefly described with reference to FIG. 16. An exposure device 1 is placed opposite to a surface of a wafer W. The exposure device 1 is provided on its head with a lens 10. The head of the exposure device 1 is provided in a part near the periphery of the lens 10 with a pouring hole 11 through which a liquid, such as pure water, for forming a liquid layer on the surface of the wafer W is poured, and a suction hole 12 through which pure water poured onto the wafer W is recovered by suction. Pure water is poured through the pouring hole 11 onto the surface of the wafer W to form a water film between the lens 10 and the surface of the wafer W. The pure water wetting the surface of the wafer W is recovered by sucking the pure water through the suction hole 12. Thus a liquid film, namely, the pure water film, is formed between the lens 10 and the surface of the wafer W. A resist film formed on the surface of the wafer W is irradiated with radiation emitted by a light source, not shown, and traveled through the lens 10 and the liquid film to form a latent image of a predetermined circuit pattern in the resist film.
Then as shown in FIG. 17, the wafer W coated with the water film is translated to align the next transfer range (shot range) 13 in the surface of the wafer W with the exposure device 1. An exposure cycle is thus repeated to transfer the predetermined circuit pattern repetitively to the resist film formed on the surface of the wafer W. In FIG. 17, shot regions 13 are shown in a size greater than their actual size.
The possibility that the wafer wetted with water drops is carried from an exposure system to a coating and developing system is a problem in the immersion exposure method. The wafer W processed by an exposure process is subjected to a heat treatment. If the wafer W is wetted with water drops or if water drops have dried and watermarks are formed on the wafer W in the heat treatment, the water drops or the watermarks affect adversely to the resolution of parts of the circuit pattern underlying the water drops or the watermarks. Therefore, the surface of the wafer needs to be cleaned after exposure to remove water drops.
However, the following problems arise in cleaning the wafer processed by the immersion exposure process. Studies have been made to form a highly water-repellent protective film that come into contact with, for example, water at a contact angle between about 70° and about 100° to assure throughput comparable top that of the known exposure system by enhancing the scanning follow-up ability of the immersed part of the exposure device (the head of the lens). However, the high repellency of the protective film increases the possibility of water drops remaining on the protective film.
As generally known, a cleaning unit for cleaning a wafer W is combined with a developing unit. Generally, the cleaning unit cleans the wafer W by a spin cleaning method that rotates a wafer W while a cleaning liquid is poured onto a central part of the wafer W, and then spin-dries the wafer W. FIGS. 18(a), 18(b) and 18(c) typically illustrate changes of the state of a cleaning liquid while a wafer is cleaned by a spin cleaning method. FIG. 18(a) shows a state of a wafer when the contact angle between the surface of the wafer and water is 70° or below and the wafer is rotated at a high rotating speed of 2000 rpm. FIG. 18(b) shows a state of a wafer when the contact angle between the surface of the wafer and water is between 70° and 110° and the wafer is rotated at a high rotating speed of 2000 rpm. FIG. 18(c) shows a state of a wafer when the contact angle between the surface of the wafer and water is between 70° and 110° and the wafer is rotated at a low rotating speed of 200 rpm.
In FIG. 18(a), a cleaning liquid 15 poured through a nozzle 14 onto a central part of a wafer W spreads toward the circumference of the wafer W and covers the surface of the wafer W entirely. Then, the nozzle 14 is moved toward the circumference of the wafer W to expand a dry region on the surface of the wafer W gradually from a central part toward the circumference of the wafer W. Even if the surface of the wafer W is not highly repellent, it is difficult to remove water drops completely from the surface of the wafer W. In other words, the adjustment of parameters including the rate of pouring the cleaning liquid, the moving speed of the nozzle 14 and the rotating speed of the wafer W is considerably difficult. When the surface of the wafer W has high repellency as shown in FIG. 18(b), a film of the cleaning liquid 15 runs about wildly when the cleaning liquid 15 spreads from a central part toward a circumferential part of the surface of the wafer W and a dry region in the surface of the wafer W expands radially outward. Consequently, a cleaning liquid film breaks and cleaning liquid drops cannot be easily removed, i.e., cleaning liquid drops do not roll radially outward on the wafer W. Consequently, the cleaning liquid drops cannot be completely removed even if the parameters are adjusted.
When the wafer W is rotated at a low rotating speed as shown in FIG. 18(c), the cleaning liquid poured through the nozzle 14 onto the wafer W moves radially outward at a low speed. Therefore, the nozzle 14 unavoidably needs to be moved at a low moving speed and processing speed is unavoidably low. Since the wafer W is rotated at a low rotating speed and centrifugal force that acts on the cleaning liquid poured onto the wafer W is low, the cleaning liquid poured onto the wafer W is splashed in all directions and, eventually, the cleaning liquid remains in drops on the wafer W in some cases.
The adjustment of the process parameters of the known spin cleaning process is difficult when the wafer is rotated at a high rotating speed. The adjustment of the process parameters becomes more difficult when the protective film is formed on the wafer because it is expected that different device makers use different types of protective films and contact angle between the protective film and the cleaning liquid changes with time. On the other hand, the cleaning process takes a long time and there is strong possibility of water drops remaining on the wafer when the wafer is rotated at a low rotating speed.
It is preferable to process a substrate coated with a film, such as a resist film or a protective film, by a cleaning process to remove substances contained in the film and capable of being eluted. However, the cleaning process needs to clean the substrate such that any small water drops do not remain on the surface of the substrate. If water drops exist on the surface of the substrate, the so-called water marks, namely, dehydrated stains, will be formed on the surface of the substrate before the substrate is transferred to the exposure unit and will cause defects during an exposure process. The protective film prevents a resist film from coming into contact with a liquid during an immersion exposure process or to enhance the repellency of the surface of the substrate.
An improved, known spin cleaning method proposed in JP-A 2001-53051 (p. 2, paragraphs 0036 and 0050) uses a gas nozzle for jetting an inert gas in addition to a nozzle for pouring a cleaning liquid. A gas is jetted through the gas nozzle at a region on which a cleaning liquid is poured so as to make the cleaning liquid flow outward. Since the gas is jetted simultaneously with pouring the cleaning liquid onto the region, the cleaning liquid poured on the region is splashed in liquid drops. The liquid drops remain on the substrate if the substrate is rotated at a low rotating speed because centrifugal force that acts on the liquid drops is low when the rotating speed is low.